Reliable ball bearings having a long life are essential to the successful operation of many rotary instruments, particularly gyroscopes used in inertial navigation. An element critical to the success of any such ball bearing is the retainer ring which in general loosely positions the individual balls for rotation between the two races or grooves of relatively rotating instrument parts. Especially in gyroscope bearings, the retainer is advantageously porous, acting as a lubricant source and supplier of the small amount necessary to lubricate the balls and races.
Various substances which have the necessary porosity have been used to form such bearing retainers in the past. Among these are laminated phenolics, a porous nylon known by the trade name Nylasint and porous polyimide.
A type of porous polyimide (See U.S. Pat. No. 3,705,118) has been developed for use as a ball bearing retainer and the retainer formed of this material is made in the following manner. Particles of cured polyimide powder are mixed with an uncured polyimide powder binder and the two are blended together. The powder is isostatically molded into a rod configuration at 1000 pounds per square inch pressure at 350.degree. temperature. The resulting rod is then cured at 500.degree. F. or C. and cut into individual discs which are machined into the desired retainer configurations.
In machining of the polyimide rod into retainers, it is very important and usually difficult, time consuming and costly to produce porous surfaces that are not sealed, especially in the ball pocket region from which the lubricant is supplied. Such sealing severely reduces the wicking action of the retainer in absorbing and dispensing the lubricant through the retainer surfaces onto the rotating balls. Therefore, the machining process requires considerable time and expense because of the intricate nature of the finished retainer and because of the care that must be taken in fabricating it.